1. Field of the Invention
The present invention relates to an ink-jet printhead and a method for manufacturing the same. More particularly, the present invention relates to an ink-jet printhead having improved efficiency and performance, and a method for manufacturing the same.
2. Description of the Related Art
Typically, ink-jet printheads are devices for printing a predetermined image, color or black, by ejecting a small volume droplet of printing ink at a desired position on a recording sheet. Ink-jet printheads are largely categorized into two types depending on which ink droplet ejection mechanism is used. A first type is a thermally driven ink-jet printhead in which a heat source is employed to form and expand bubbles in ink causing ink droplets to be ejected. A second type is a piezoelectrically driven ink-jet printhead in which a piezoelectric material deforms to exert pressure on ink causing ink droplets to be ejected.
Hereinafter, the ink ejection mechanism in the thermally driven ink-jet printhead will be described in greater detail. When a pulse current flows through a heater formed of a resistance heating material, the heater generates heat and ink adjacent to the heater is instantaneously heated to about 300° C., thereby boiling the ink. The boiling of the ink causes bubbles to be generated, expand, and apply pressure to an interior of an ink chamber filled with ink. As a result, ink near a nozzle is ejected from the ink chamber in droplet form through the nozzle.
The thermal driving method includes a top-shooting method, a side-shooting method, and a back-shooting method depending on a growth direction of bubbles and an ejection direction of ink droplets.
The top-shooting method is a method in which the growth direction of bubbles is the same as the ejection direction of ink droplets. The side-shooting method is a method in which the growth direction of bubbles is perpendicular to the ejection direction of ink droplets. The back-shooting method is a method in which the growth direction of bubbles is opposite to the ejection direction of ink droplets.
The ink-jet printheads using the thermal driving method should satisfy the following requirements. First, manufacturing of the ink-jet printheads should be simple, costs should be low, and should facilitate mass production thereof. Second, in order to obtain a high-quality image, cross talk between adjacent nozzles should be suppressed while a distance between adjacent nozzles should be narrow; that is, in order to increase dots per inch (DPI), a plurality of nozzles should be densely positioned. Third, in order to perform a high-speed printing operation, a period in which the ink chamber is refilled with ink after being ejected from the ink chamber should be as short as possible and the cooling of heated ink and heater should be performed quickly to increase a driving frequency.
FIGS. 1 through 4 illustrate various structures of conventional ink-jet printheads using the back-shooting method.
FIG. 1 illustrates a separated perspective view of a conventional ink-jet printhead. Referring to FIG. 1, the ink-jet printhead has a structure in which a substrate 36, on which a nozzle 32 and an ink chamber 34 are formed, is stacked on an ink reservoir 30, in which an ink supply conduit 31 is formed. In this printhead, a heater is disposed around the nozzle 32, although the heater is not shown in FIG. 1.
In the above structure, when a pulse current is applied to the heater and the heater generates heat, ink in the ink chamber 34 is boiled, and bubbles are generated. The bubbles expand continuously and apply a pressure to ink in the ink chamber 34. This pressure causes ink to be ejected in droplet form through the nozzle 32.
In the ink-jet printhead using the back-shooting method, in order to effectively use energy of a bubble in a direction of ink ejection, flow resistance should be large so that the flow of ink is suppressed in a direction of bubble growth.
However, an element of the printhead for creating flow resistance between the ink chamber 34 and the ink reservoir 30 does not exist in the aforementioned ink-jet printhead. Accordingly, flow in the direction of bubble growth cannot be restricted. Thus, a larger amount of energy is required to be generated in the direction of bubble growth in order to eject ink. In addition, since a height of the ink chamber 34 is almost the same as a thickness of the substrate 36, a size of the ink chamber 34 is increased unless a very thin substrate is used. As a result, an amount of ink affected by bubbles is increased. This means that an inertia force of ink is increased, and an operating frequency of the printhead is restricted by the inertia force of ink.
FIG. 2 illustrates a cross-sectional view of a structure of another conventional ink-jet printhead. Referring to FIG. 2, a nozzle 42 is formed at one end of an ink channel 40 through which ink flows, and a heater 44 is disposed around the nozzle 42. The ink channel 40 has a shape such that a sectional area thereof gradually increases in a direction of bubble growth.
In the aforementioned ink-jet printhead, flow resistance is reduced in the direction of bubble growth. Accordingly, a larger bubble energy is required to eject ink.
FIG. 3 illustrates a cross-sectional view of another structure of a conventional ink-jet printhead. Referring to FIG. 3, a substantially hemispheric ink chamber 50 is formed in a substrate 65, and a manifold 54 for supplying ink to the ink chamber 50 is formed under the substrate 65. An ink channel 52 for providing communication between the ink chamber 50 and the manifold 54 is formed on a bottom center of the ink chamber 50. A nozzle plate 60, in which a nozzle 58 is formed, is stacked on a top surface of the substrate 65. The nozzle plate 60 forms an upper wall of the ink chamber 50. A heater 56 is formed in the nozzle plate 60 and surrounds the nozzle 58.
FIG. 4 illustrates a cross-sectional view of a structure of yet another conventional ink-jet printhead. Referring to FIG. 4, an ink chamber 72, which has a substantially hemispherical shape and is to be filled with ink, and an ink channel 74, which is formed to a smaller depth than the ink chamber 72 and supplies ink to the ink chamber 72, are formed on a surface of a substrate 70. A manifold 76 for supplying ink to the ink channel 74 is formed on a bottom surface of the substrate 70. A nozzle plate 80 formed of a plurality of material layers is stacked on an upper surface of the substrate 70 and forms an upper wall of the ink chamber 72. A nozzle 78, through which ink is ejected, is formed in a position of the nozzle plate 80 corresponding to a center of the ink chamber 72. A ring-shaped heater 82 is formed around the nozzle 78 and surrounds the nozzle 78. A nozzle guide 84 is additionally formed in this printhead. The nozzle guide 84 guides an ejection direction of ink and ejects ink droplets to be precisely perpendicular to the upper surface of the substrate 70.
As described above, the conventional ink-jet printheads shown in FIGS. 3 and 4 have a structure in which a manifold is formed between an ink channel and an ink reservoir. However, in the previous ink-jet printhead, it is not easy to process an ink channel. In addition, even though the ink channel may be processed, there is a limitation on a shape of the ink channel or there may be an error between processed ink channels.
When the ink channel is processed on the substrate, there is a limitation on the shape of the ink channel. More specifically, the shape of the nozzle is transferred to the shape of the ink channel using a method of processing an ink channel on the substrate. In general, flow resistance of a conduit is proportional to a length of the conduit and is inversely proportional to the square of a sectional area of the conduit. Flow resistance can be adjusted by adjusting the length of the conduit. However, it is difficult to adjust a flow resistance ratio of a nozzle and an ink channel that determine the performance of the ink-jet printhead using the back-shooting method because of requirements on those dimensions. Specifically, the length of the nozzle should be sufficiently long so that ink is stably ejected. In this case, the length of the ink channel should be sufficiently long. If the ink channel is processed through the nozzle, a processing time is increased. In addition, as the processing time is increased, the etching amount of a passivation layer formed under a heater is gradually increased. Thus, the thickness of the passivation layer should be excessively large.
When the ink channel is processed under the substrate, due to a step of a manifold, it is difficult to process the ink channel, and even though the ink channel may be processed, there may be an error between processed ink channels. In addition, the depth of the manifold is generally greater than 400 μm. In a structure having a large step, it is difficult to perform a photolithography process using an existing semiconductor device. First, when coating a photoresist, a photoresist that can be plated should be used, or a specific device, such as a spray coater, should be used. When exposing the photoresist, a specific device, such as a reconstructed projection aligner, and not a general exposure device, should be used. Further, even though the ink channel is processed using the aforementioned method, there is a larger error than in processing in which there is no step of the manifold. Since flow resistance is inversely proportional to the square of a sectional area of a conduit, even a small error in processing of the ink channel affects the performance of the ink-jet printhead.